1. Field of the Invention
The present application is directed to the field of metrology instruments and corresponding techniques, and more particularly to metrology tools capable of sub-surface imaging.
2. Description of Related Art
Despite broad demand from semiconductor, materials, and biomedical communities, no existing technique provides routine broadly adopted nanoscale sub-surface metrology. Surface metrology is the science of measuring small-scale features on surfaces. Surface primary form, surface waviness, and surface roughness are exemplary parameters that are measured. Sub-surface metrology is the study of parameters defining the constitution of a solid below the surface. One example of sub-surface metrology may include detecting sub-surface defects in silicon wafers. Although systems exist for sub-surface metrology, each had limitations that prevent broad application of the systems and methods, as discussed in further detail below.
Existing techniques for performing sub-surface metrology are varied, although each is associated with particular limitations. A transmission electron microscope (TEM) can see a small distance through materials of low electron density, revealing some sub-surface details. However, this technique requires destructive and time consuming thinning of samples, limiting its applicability. Scanning laser confocal microscopy can provide resolution down to roughly 200 nm, but can only be used on samples that are optically transparent. Acoustic microscopy, for example manufactured by Sonoscan, is employed very successfully for a wide range of sub-surface metrology applications. However, the lateral resolution is limited by the wavelength of the ultrasonic excitation, typically in the range of millimeters to microns. Projection x-ray systems, for example manufactured by Xradia or Skyscan, have made dramatic strides in recent years in sub-surface microscopy. However, when applied at the nanoscale, these techniques are still destructive, requiring samples that are diced to less than 25 mm across and thinned to less than 100 um in thickness. Further, the use of high energy ionizing x-ray radiation is also a consideration for many applications, where the x-rays can crosslink and/or otherwise damage samples. Pulsed thermal microscopy is another technique that provides sub-surface information by flash irradiating a sample with IR radiation and then imaging the heat re-emitting from a sample. This technique provides contrast for materials with different thermal properties, but still suffers from optical resolution limits.
Among all the techniques discussed so far, Atomic Force Microscopy (AFM) has a dramatic advantage because if can routinely produce sub-100 nm resolution, non-destructively, on a wide range of samples ranging from semiconductors, to material science, to biomedical samples. However, standard AFM imaging techniques are only sensitive to surface features or features a few nanometers below the surface. Variations of AFM that attempt to combine some of the advantages of AFM and acoustic microscopy, such as ultrasonic force microscopy and atomic force acoustic microscopy, have been evolving over the past decade. In 2005, a paper in Science described a technique called Scanning Near-field Ultrasound Holography (SNFUH). In this paper, the authors demonstrated the ability to detect and resolve 20 nm diameter nanospheres buried 500 nm below a sample surface. The paper also demonstrated the ability to observe buried voids in a semiconductor sample and malarial parasites within a red blood cell.
Despite the above listed and described systems and the potential of SNFUH, the current state of the SNFUH technique makes it completely impractical for broad adoption in industry. First, measurements have only been demonstrated on cm-scale samples, with areas more than 500× smaller than a conventional 300 mm silicon wafer. Second, to achieve sufficient contrast, the current SNFUH technique requires that an ultrasonic actuator be individually bonded to each sample. To achieve broad adoption, the semiconductor industry requires the ability to rapidly measure multiple sites (for example 15 sites) distributed across a wafer with throughput in the range of 20+/wafers per hour. There is currently no feasible technique to bond and debond ultrasonic actuators to meet these throughput requirements. As such, current SNFUH techniques are not conveniently scalable to handle large samples. Further, the semiconductor industry also places strict limits on backside particle contamination, a goal unlikely to be achieved with the current ultrasonic coupling requirements. For example, current industry standards limit the maximum number of particles added per wafer pass to less than 1000. (These standards also get tighter with each semiconductor generation.) This includes particles from all sources including wafer handling and contact between the sample holder and the wafer. Finally, the current SNFUH technique required Ph.D. level expertise with a substantial amount of experimentation to obtain a high quality image. The SNFUH introduced an ultrasonic source by driving the cantilever probe at the fixed end. The cantilever, having a length of hundreds microns, can couple Mhz ultrasonic waves to the tip effectively but will fail to couple Ghz ultrasonic wave due to attenuation of the Ghz wave along the cantilever. The architecture of the SNUHF limits its use to acoustics of Mhz frequency range
What is needed is a system and method for facilitating nondestructive imaging of critical buried nanostructures with resolution exceeding 20 nm laterally and better than 1 nm vertically, including resolution of features buried more than 100 nanometers below the sample surface. What is further needed is such a system and method configured to allow such imaging over an arbitrary sample size with improved throughput. What is yet further needed is such a system and method configured to provide for minimal front and backside contamination.